Fix PR_CI_Document_Preview Job (#17514)

* test=develop

* test=develop

tools/document_preview.sh

 #!/bin/bash
-PADDLE_ROOT=/paddle
+PADDLE_ROOT=/home
+mkdir ${PADDLE_ROOT}
 cd ${PADDLE_ROOT}
+pip install /paddle/build/opt/paddle/share/wheels/*.whl
 git clone https://github.com/PaddlePaddle/FluidDoc
 git clone https://github.com/tianshuo78520a/PaddlePaddle.org.git
-sh ${PADDLE_ROOT}/FluidDoc/doc/fluid/api/gen_doc.sh
-pip install ${PADDLE_ROOT}/build/opt/paddle/share/wheels/*.whl
+cd ${PADDLE_ROOT}/FluidDoc/doc/fluid/api
+sh gen_doc.sh
 apt-get update && apt-get install -y python-dev build-essential
 cd ${PADDLE_ROOT}/PaddlePaddle.org/portal
 pip install -r requirements.txt